Photocatalytic Oxidation of NO₂ on TiO₂ : Evidence of a New Source of N₂O₅

N₂O₅ is an important intermediate in the atmospheric nitrogen cycle. Using a flow tube reactor, N₂O₅ was found to be released from the TiO₂ surface during the photocatalytic oxidation of NO₂, revealing a previously unreported source of N₂O₅. The rate of N₂O₅ release from TiO₂ was dependent on the initial NO₂ concentration, relative humidity, O₂/N₂ ratio, and irradiation intensity. Experimental evidence and quantum chemical calculations showed that NO₂ can react with the surface hydroxyl groups and the generated electron holes on the TiO₂, followed by combining with another NO₂ molecule to form N₂O₅. The latter was physisorbed on TiO₂ and had a low adsorption energy of −0.13 eV. Box model simulations indicated that the new source of N₂O₅ released from TiO₂ can increase the daytime N₂O₅ concentration by up to 20 % in urban areas if abundant TiO₂-containing materials and high NOx concentrations were present. This joint experimental/theoretical study not only demonstrates a new chemical mechanism for N₂O₅ formation but also has important implications for air quality in urban areas. © 2023 Wiley-VCH GmbH